It is well known that polyethylene terephthalate is one of the most useful synthetic fibers and is widely used as film in industrial uses and for packaging foods. Recently polyethylene terephthalate has come into rapid use in biaxially stretched bottles for packaging liquid foods and cosmetics. This is attributed to the fact that, as compared with polyvinyl chloride or polyolefin-EVA (ethylene-vinyl alcohol) copolymer, double-layered material conventionally used for bottles, polyethylene terephthalate is excellent in hygienic properties, impact resistance, heat resistance, transparency, gas barrier properties, chemical resistance, weatherability, etc., and has well balanced physical properties for bottles.
However, polyethylene terephthalate is essentially a highly crystalline resin and, therefore, appears opaque. Hence, various techniques have been employed to obtain transparent bottles of polyethylene terephthalate.
In general, polyethylene terephthalate bottles (hereinafter abbreviated "PET bottles") are manufactured by blow molding techniques such as direct blow molding, injection blow molding or biaxially stretching blow molding. In order to manufacture transparent PET bottles, it is important, from the viewpoint of material aspects, to select polyethylene terephthalate having a proper molecular weight according to the blow molding method and the section thickness of the molding to be manufactured and, from the viewpoint of molding aspects, it is necessary to preliminarily dry the PET chips well or to rapidly cool the resin in the molten state as fast as possible. In particular, with thick-walled bottles, it is of importance to minimize crystallization of polyethylene terephthalate in the period between parison molding and blow molding.
As one approach for modifying polyethylene terephthalate to satisfy the above requirements, it is known to use a small quantity of isophthalic acid together with terephthalic acid as the dicarboxylic acid component of PET or to use a small quantity of neopentylglycol or cyclohexanedimethanol together with ethylene glycol as the glycol component of PET, thus producing copolymeric PET having a slow crystallizing rate by copolymerizing the above-described ingredients (for example, see Lecture Abstracts of the 11th Colloquium on Structure and Physical Properties of High Polymers, held by the Japanese High Polymer Society, Kanto Branch, on June 16, 1981, "Recent Progress in Modification of Polymers", p. 3).
With the above situation in mind, the inventors intensively investigated glycols functioning as comonomers which provide a PET crystallization-controlling effect and, as a result, found that 2-methyl-1,3-propanediol is extremely excellent as such a glycol, thus having achieved the present invention.